This invention relates generally to a hydraulic fracturing process for a subterranean formation, which comprises introducing into the formation under pressure a fracturing fluid comprising solid particulate suspended in a fluid dispersion comprising water, a specified gaseous or supercritical component, and a surfactant component selected from a certain class of polysaccharide compounds. With respect to the formation of a three-component fluid dispersion, the invention is related to the process claimed in U.S. Pat. No. 4,856,588, issued Aug. 15, 1989, entitled "Selective Permeability Reduction of Oil-Free Zones of Subterranean Formations".
U.S. Pat. No. 4,856,588 describes and claims an improved process for the recovery of oil from a formation which has both zones of high oil saturation and zones substantially free of oil. That process involves the introduction into a substantially oil-free zone of a dispersion of water, a gaseous or supercritical component, and a polysaccharide surfactant. The dispersion selectively reduces the permeability of flow through these zones, without plugging high oil-saturation zones of the formation.
The present application describes and claims a closely-related process in which the same fluid dispersion, together with suspended solid particulate, is injected into a subterranean formation for purposes of hydraulic fracturing.
Hydraulic fracturing is a well known process wherein the permeability of a subterranean formation is increased by generating high permeability cracks in the formation rock. Particulate (commonly sand) suspensions are injected at sufficiently high rates and pressures to generate these cracks, called fractures, in the rock. The fractures are held open by the particulate, called proppant. The fractures increase the permeability of the formation for the flow of oil and gas, and thus enhance its recovery.
Although it is more common to use a fracturing fluid which is a solution of crosslinked polymer in water, the use of surfactant foams in hydraulic fracturing has been known for several years, particularly in applications to low pressure formations and in water-sensitive formations. Surfactant foams are known to have advantages in combining a relatively low viscosity with good proppant suspension and transport capabilities. It is also recognized that the foams carry a minimal amount of liquid phase in the formation. Still further, foams do not suffer, at least to the same extent as aqueous fluids, from problems such as permeability damage to the formation which may result as liquids leak off from the fracture into the formation.
In one important aspect, the present invention utilizes as the surfactant component of the fracturing fluid one or more of certain polysaccharide compounds. These polysaccharides are nonionic surfactants which are generally known for use as components of laundry detergents, personal cleaning products, dishwashing formulations, fire fighting foams, ore mining and treatment solutions, etc. Suggestions have been made in the prior art for use of alkyl polysaccharides, and particularly alkylglycosides, as a component of well drilling fluids, that is, the fluids circulated down a well bore during drilling to cool the drill bit and suspend and carry rock cuttings to the surface. U.S. Pat. No. 4,683,074 has recently described the use of particularly defined mixtures of monoglycoside and polyglycoside surfactants in acid containing compositions intended for use in metal and porcelain cleaner formulations and in formulations for oil well acidizing and acid fracturing. Acid fracturing and acidizing processes operate under different mechanisms than hydraulic fracturing processes. Acidizing processes use a strongly acidic composition to dissolve rock or etch channels into the rock in a subterranean formation to enhance its permeability. The process of U.S. Pat. No. 4,683,074 employs polysaccharide surfactants because of their excellent stability to highly acid conditions. The patent mentions an acidizing foam, but does not identify its composition or properties. As is recognized in the art (e.g., the text Reservoir Stimulation, H. J. E. Economides and K. G. Nolte, Schlumutilize neutral to mildly basic fluids and are readily distinguishable from acidizing processes.